The production of liquefied gas usually consists of gas compression, cooling, and expansion through a value to produce a cryogenic liquid. This basic Joule-Thompson cycle is generally used today, but, because the expansion through a valve is at constant enthalpy and thus zero isentropic efficiency, greater liquid production could be achieved with a device that can extract work from the expanding fluid. Expansion engines and expansion turbines have been used extensively to expand gases within their superheat region, but few applications have been extended into the liquid region.
The problem of operating an expansion with a super critical fluid is the fact that the fluid is single phase and behaves with compressibility similar to water, i.e., it has little to no compressibility, and therefore, results in the large pressure pulsations in an expansion engine inlet line.